1. Field of the Invention
This invention relates to photodiodes and more particularly to a p-i-n junction photodiode reactive to the visible blind and true solar blind spectral regions.
2. Description of the Related Art
Solid state photoconductors and photodiodes constitute an important class of photodetectors which convert electromagnetic energy directly into electric energy via the photoconductivity effect that occurs in semiconductors. Recent improvements in semiconductor material quality has focused development on photoconductors and photodiodes with peak sensitivity in the visible blind and true solar blind spectral regions. True solar blind detection requires sensitivity to wavelengths below approximately 300 nm, while visible blind detection requires sensitivity to wavelengths below approximately 400 nm. The development of true solar blind detectors has proven particularly important. It is known that the earth's ozone layer filters out light with a wavelength below approximately 300 nm. If a true solar blind detector can be developed, light sources below the earths atmosphere (such as missile plumes) can be detected while rejecting solar background interference. This technology has many other applications, such as detection in a biological absorption spectrum, detection of non-visible hydrogen flames and satellite communication.
Various visible blind photoconductors have been successfully developed with high intrinsic gain and low leakage currents. [M. A. Khan, et.al., High-responsivity Photoconductive Ultraviolet Sensors Based on Insulating Single-Crystal GaN Epilayers, Appl.Phys.Lett.60 (23), Jun. 8, 1992, Pages 2917-2019; J. C. Carrano, et.al., Very Low Dark Current Metal-Semiconductor-Metal Ultraviolet Photodetectors Fabricated on Single-Crystal GaN Epitaxial Layers, Appl.Phys.Lett.70 (15), Apr. 14, 1997, Pages 1992-1994]. Visible blind photodiodes have also been developed using GaN as the active device layer resulting in a wavelength detection cutoff of approximately 365 nm. Visible-blind Schottky barrier photodiodes have been demonstrated with responsiveness as high as 0.18 Amps/Watt (A/W) and response time of 118 nanoseconds (ns) [Q.Chen et.al., Schottky Barrier Detectors on GaN for Visible-Blind Ultraviolet Detection, Appl.Phys.Lett.70 (17), Apr. 28, 1997, Pages 2277-2279]. Visible blind Schottky photodiode arrays with 8.times.8 pixels have also been demonstrated, with somewhat poorer characteristics, having responsiveness of 0.05 A/W with a response time of 50 ns. [B. W. Lim et.al., 8.times.8 GaN Schottky Barrier Photodiode Array for Visible Blind Imaging, Electronics Letters, Vol.33 No.7, 1997, Page 633]. Visible blind p-i-n photodiodes have also been developed with responsiveness as high as 0.12 A/W and response times of 12 ns. [G. Y. Xu, et.al., High Speed, Low Noise Ultraviolet Photodetectors Based on GaN p-i-n and AlGaN(p)-GaN(i)-GaN(n) Structures, Appl.Phys.Lett.71 (15), Oct. 13, 1997, Pages 2154-2156]. Silicon carbide (SiC) visible blind photodiodes have been developed having high responsivity and low dark current. [D. M. Brown et. al., Silicon Carbide UV Photodiodes, IEEE Trans. On Electr. Devices, Vol.40 No.2, Feb. 1993, Pages 325-331]. Si photodiodes cannot be made intrinsically visible blind without the use of external filters due to the electronic band structure of silicon.
The development of true solar blind photodetectors has proven more difficult. Attempts have been made to modify Si photodiodes to make them true solar blind photodiodes. However, this modification requires the addition of external filters to the photodiodes. These filters are complex, expensive, fragile and typically allow only approximately 50% transmission of the light source to the device. True solar blind photoconductors without external filters have also been developed [D. Walker et.al., AlGaN Ultraviolet Photoconductors Grown on Sapphire, Appl.Phys.Lett.68 (15), Apr. 8, 1996, Pages 2100-2101; B. W. Lim, et.al., High Responsivity Intrinsic Photoconductors Based on Al.sub.x Ga.sub.1-x N, Appl.Phys.Lett.68 (26), Jun. 24, 1996, Pages 3761-3762], but these devices suffer consistently slow response times and degraded performance compared to visible blind photoconductors.
Efforts have been made, largely unsuccessfully, to develop true solar blind nitride based p-i-n photodiode It is well known that, as the wavelength of light decreases, the energy increases. To be solar blind, the bandgap in the active i-region of the photodiode must be large enough to be responsive to the energy contained in an approximately 300 nm wavelength light source, while rejecting longer wavelength light. The addition of aluminum (Al) to a nitride based semiconductor material will increase the bandgap of the material. The greater the percentage of Al, the larger the bandgap. For the active i-region to have a large enough bandgap to be responsive to wavelengths lower than 300 nm, a relatively high percentage of Al must be added.
In most p-i-n photodiodes, the light incident on the device illuminates the p-type region first. The light to be detected in the active i-region must be able to first pass through the p-type region. However, conventional p-i-n photodiodes are homojunction, having the same bandgap for all regions. If the p-type region has the same bandgap as the i-region, the light to be detected in the i-region will excite carriers in the p-type region and will be absorbed, so it cannot pass to the i-region for detection. A p-type region with the same bandgap as the i-region effectively blocks light from passing to the i-region. This problem becomes worse when Al is added to the i-region to enlarge its bandgap energy.
A proposed solution to this problem was to increase the bandgap in the p-type region to an energy level greater than the bandgap of the i-region (heterostructure) so that the wavelength of light to be detected by the active i-region will not have sufficient energy to excite the higher bandgap p-type region. The goal is to have the light pass through the p-type region without being absorbed so that is can be detected in the i-region. To obtain a higher bandgap, the p-type region must have a greater percentage of Al than the i-region.
However, published research indicates that high Al-content AlGaN alloys present major challenges in terms of growth, lattice mismatch strain, and doping, particularly for p-type AlGaN [M. Suzuki, et.al., Doping Characteristics and Electrical properties of Mg-doped AlGaN Grown by Atmospheric-Pressure MOCVD, 2.sup.nd ICNS, Oct. 27-31 1997, Pages 464-465]. During the growth of a high Al content p-type GaN diode, lattice mismatch will result in tensile strain at the junctions and eventual cracking. High Al content n-type GaN with good structural integrity has been grown with greater success than high Al content p-type GaN.
In addition, high Al content AlGaN can lead to significantly increased response times and reduced responsiveness in photoconductors [Q.Chen et.al., Schottky Barrier Detectors on GaN for Visible-Blind Ultraviolet Detection, Appl.Phys.Lett.70 (17), Apr. 28, 1997, Pages 2277-2279, and references therein]. Photodiodes with high Al content p and n-type regions will likely experience the same increased response time and reduced responsiveness.